The fabrication of high-performance thin-film transistors (TFTs) by solution phase processes is a promising approach to developing commercially viable, low-cost, large-area electronics. Despite intense efforts to develop solution-processed organic semiconducting films, the highest field-effect mobilities (μFETs) for organic TFTs (OTFTs) reported to date are ˜1.0 cm2 V−1 s−1 (p-type, small molecule), 0.21 cm2 V−1 s−1 (n-type, small molecule), ˜0.6 cm2 V−1 s−1 (p-type, polymer), and 0.1 cm2 V−1 s−1 (n-type, polymer). These values are lower than those of optimized OTFTs fabricated with vapor-deposited or single-crystal organic semiconductors by up to ˜10×. These observations suggest that conventional OTFTs will be useful for low- or medium-performance applications but not for solution-processed high-speed circuits. Consequently, inorganic semiconductors emerge as potential candidates since they can exhibit bulk field effect mobilities in excess of 100 cm2 V−1 s−1.
However, inorganic semiconductors are typically intractable in common solvents so that soluble precursors must be employed, then converted to the active semiconducting films. This approach typically requires noxious solvents and high annealing temperatures (>500° C.) to achieve sufficient film crystallinity and texturing (for crystalline semiconductors) for acceptable charge transport properties. High annealing temperatures are incompatible with inexpensive plastic substrates. Furthermore, when bottom-gate transistors are fabricated, the gate insulator must be sufficiently robust to survive the processing conditions and thin enough to ensure low operating voltages. Thus, these requirements significantly limit the temperatures at which the semiconductor films can be annealed and/or the choice of materials for the gate dielectric. For example, ultra-thin inorganic films deposited from solution are invariably leaky and morphologically very rough. Meanwhile, many known polymeric dielectric materials are not sufficiently stable, whether thermally and/or mechanically, to withstand high annealing temperatures and aggressive aqueous solutions. Furthermore, and not obvious a priori, the dielectric surface must nucleate inorganic semiconductor film growth from solution when bottom-gate TFTs are fabricated. As such, to date, TFTs have not been fabricated using both a solution-processed inorganic semiconductor and a solution-processed gate dielectric.
Recently, several metal oxides have emerged as promising semiconductors for low-temperature processed TFTs. These include indium oxide (In2O3), zinc oxide (ZnO), tin dioxide (SnO2), indium-gallium oxides (IGO), amorphous zinc-tin oxides (a-ZTO), amorphous indium-zinc oxides (a-IZO), amorphous indium-gallium-zinc oxides (a-IGZO), and amorphous cadmium-indium-antimony oxides. Polycrystalline oxides like ZnO have columnar grain structures even when deposited at room temperature. Their films inevitably suffer from problems associated with large densities of grain boundaries, including instability to air due to O2/H2O/CO2 chemisorption and film surface roughness due to facet formation.
Amorphous metal oxide films generally exhibit more uniform microstructures and smoother surfaces than crystalline oxides, which in turn yield better adhesion to the substrate. However, these films are usually deposited from the vapor phase, using methods such as sputtering and pulsed-laser deposition. Combinatorial approaches have been utilized to find high-mobility and low carrier doping compositions useful for TFT applications. For example, amorphous metal oxides including In—Ga—Zn—O, In—Zn—O, Zn—Sn—O, and In—Sn—O have been investigated for TFT applications. However, in these studies, the different vapor pressures of the individual element precursors often lead to difficulties in tuning the optimum film composition and, importantly, reproducibility problems. In addition, low-pressure deposition processes are expensive to scale for large areas and high throughput. Meanwhile, previous metal-oxide-based TFTs fabricated by solution deposition generally exhibit poor performance, in particular low field-effect mobilities, low Ion:Ioff ratios and large operating voltages, any of which tend to preclude most practical applications.
Accordingly, there is a need in the art for new formulations for processing inorganic semiconductor materials in solution and associated methods for fabricating inorganic (e.g., metal oxide and chalcogenides) TFTs by semiconductor solution deposition techniques. In addition, the need to develop high-performance TFTs from solution and annealed at temperature <500 C as well as to further explore organic dielectric compatibility with solution-processed high-μFETs inorganic semiconductors remains an ongoing concern in the art.